1. Field of the Invention
The present invention relates to an information-reproducing apparatus for reproducing digital data recorded on an information-recording medium on the basis of a reproducing clock generated by a PLL (Phase Locked Loop) circuit.
2. Related Background Art
FIG. 18 is a block diagram showing an example of a data recording/reproducing apparatus such as an optical apparatus. Reference numeral 1 in FIG. 1 denotes an optical disc, which is an information recording medium, 2 denotes a spindle motor for rotating the optical disc 1 at a constant speed, 3 denotes a pickup for irradiating the optical disc 1 with a light beam, receiving a reflected light from the optical disc 1 to perform photoelectric conversion, and detecting information of information track on the optical disc 1 as a reproduced signal.
Reference numeral 4 denotes an amplifier for amplifying an output from the pickup 3, 5 denotes an analog-to-digital (A/D) converter for converting an output from the amplifier 4 into a digital value, and 6 denotes a phase difference detector where a digital reproduced signal converted at the analog-to-digital converter 5 is input and a reproducing clock, which is an output from VCO (voltage controlled oscillator) 9 is supplied for detecting a phase difference between the digital reproduced signal and the reproducing clock. Reference numeral 7 denotes a loop filter, to which a phase difference that is an output from the phase difference detector 6 is input. Loop filter 7 stabilizes loop characteristics of PLL loop consisting of the analog-to-digital converter 5, phase difference detector 6, loop filter 7, digital-to-analog converter 8, and VCO 9 and cuts unnecessary high-pass component.
Reference numeral 8 denotes a digital-to-analog converter which converts an output from a loop filter into analog voltage, and 9 denotes a voltage controlling oscillator (so-called VCO) in which an oscillating frequency fluctuates according to an output from the digital-to-analog converter 8. Reference numeral 10 denotes a data separator for processing a digital reproduced signal, which was digitalized at the analog-to-digital converter 5, and outputting the signal as a binary signal. A data recorded on the optical disc 1 is usually modulated appropriately to disc characteristics. For example, the optical disc 1 records modulated data such as 1-7 modulation. Reference numeral 11 denotes a demodulator for demodulating the 1-7 modulation, and 12 denotes ECC (Error Correction Code) block for decoding Reed-Solomon code, which corrects an error in demodulated data.
Next, a method for detecting phase difference in PLL will be described with reference to FIG. 19. (a) of FIG. 19 denotes a reproducing clock of the VCO 9, and (b) of FIG. 19 denotes a reproduced signal reproduced at the optical disc 1 and amplified at the amplifier 4. A sample value taken by the analog-to-digital converter 5 at an edge point of a reproduced signal shown in (b) of FIG. 19 will be a phase difference signal (a sample point designated by an arrow in FIG. 19).
As shown as (b) of FIG. 19, an edge of a reproduced signal inclines. When a sample timing of the analog-to-digital converter 5 occurs at the center of this inclination, a phase difference between the clock and the reproduced signal is zero. For example, if a reproduced signal delays, a sample value taken by the analog-to-digital converter 5 is less than that taken at the point of phase difference zero; if a reproduced signal leads, a sample value taken by the analog-to-digital converter 5 is greater than that taken at the point of phase difference zero.
According to this principal, a phase difference between the clock and the reproduced signal is detected, and the detected phase difference signal is supplied through the loop filter 7 and the digital-to-analog converter 8, which have appropriate frequency characteristics and gains, to the control voltage of VCO 9. When the phase difference detector 6 detects any delay of a reproduced signal at this moment, oscillating frequency of VCO 9 is decreased. When a phase of a reproduced signal leads, oscillating frequency of VCO 9 is increased. PLL loop operates in this manner.
Recent advance in technology of manufacturing highly denser optical discs necessitates a high accuracy of a PLL circuit for reproducing a reproducing clock from a reproduced signal. As information on an optical disc increases in density, a signal recorded on the optical disc, i.e., recording mark, becomes finer, which highly reduces the quality of a reproduced signal. Variations in quality of reproduced signals for each optical disc or within a single optical disc adversely affect PLL performance.
As amplitude of a signal changes, amplitude of a reproduced signal waveform shown as (b) of FIG. 19 changes, and sensitivity of a phase difference signal to be detected also changes. As a result, a loop gain of PLL changes. At worst, a loop becomes too unstable to keep following PLL, or tractability of a loop degrades and lowers synchronism between the clock and the reproduced signal, which causes high frequency of errors in the data separator 10.
There is variation in not only amplitude of reproduced signal but also inclination of an edge of a reproduced signal depending on variations in quality of optical discs, variations in laser powers, or an error in controlling an optical spot against an information track such as in focus tracking. A change in inclination of an edge is represented as changes in sensitivity of phase difference detection, similarly as changes in amplitude of a reproduced signal, which result in an unstable PLL as described above. Control voltage-to-frequency change characteristics of VCO 9, i.e., a gain of VCO also have wide variations from oscillator to oscillator, which causes unstable PLL.
FIG. 20 is a board diagram showing loop transfer characteristics of PLL. In FIG. 20, a curve designated by reference character “b” shows the case of an appropriate loop characteristic, a curve “a” shows the case of a low loop gain with low PLL tractability, and a curve “c” shows the case of a high loop gain with unstable PLL. In general, the higher-density an optical disc is recorded, the worse S/N and jitter for a reproduced signal in comparison with the lower-density recording, thus, the worse a tractability of PLL.
FIG. 21 is a block diagram showing an information-reproducing apparatus proposed in Japanese Patent Application Laid-Open No. 6-76486. This application discloses a configuration for changing a time constant of PLL for at starting point and for stationary state. The same numerals designate the same parts in FIG. 21 as those in FIG. 18. In FIG. 21, 7A and 7B denote loop filters. When the loop filter 7A is selected with a switch 13, it will make PLL with a small time constant of a loop characteristic for an entire PLL and a characteristic of high-speed response. When the loop filter 7B is selected with the switch 13, it will make PLL with a large time constant of a loop characteristic for an entire PLL and high stability.
Usual optical disc apparatus moves a pickup to a data area storing data to be reproduced (seek operation), locate the start of the data to be reproduced, start an PLL operation at the beginning of the first data. Then, when PLL locks the data is ready to be reproduced. Most optical disc apparatuses, which use “sector” as a unit of data to be recorded and reproduced, start a PLL operation at the beginning of this sector, and after PLL locked, reproduce the data. Thus, PLL, which can quickly lock after starting the operation, is required.
Dust or a scratch on a surface of a disc often affects a reproduced signal, which in turn will be a disturbance to PLL. This disturbance of PLL is caused by external factors. Therefore, it is also needed for PLL to have a characteristic for protecting its operation against such a disturbance due to the external factors. For this reason, Japanese Patent Application Laid-Open No. 6-76486 discloses forming PLL with quick response and a small time constant by selecting the loop filter 7A at starting PLL for the first time for a sector, and forming PLL with high stability and a large time constant by switching to the loop filter 7B after the PLL is locked.
However, in the structure of Japanese Patent Application Laid-Open No. 6-76486, characteristics of a PLL circuit have already fixed when the optical disc apparatus is designed, thus, it cannot deal with variations in amplitude of a reproduced signal and inclination of an edge caused by variations in quality of optical discs. This results in various loop gains of PLL, and at worst, it caused problems where a loop becomes too unstable to keep following PLL, or tractability of a loop degrades and lowers synchronism between a clock and a reproduced signal, which causes high frequency of errors in a data separator.